Electron tube



Au s, 1939.

L. B. HEADRICK ELECTRON TUBE Filed March 5, 1936 INVENTOR LEW/5 BAR/VA flEADR/C/f ATTORNEY Patented Aug. 8, 1939 PATENT OFFICE 2,169,040 ELECTRON TUBE Lewis Barnard Headrick, West Orange, N. J.,. assignor to Radio Corporation oi. America, a corporation of Delaware Application March 5, 1936, Serial No. 67,249

8 Claims. (01. 250-164) This invention relates to electron tubes, and more particularly to tubes of the cathode ray or- X-ray type. More particularly, the invention is directed to ways and means and method of providing for a more effective utilization of the beam 01' electrons which is developed within the tube.

It is an object of this invention to provide ways and means for securing an eflicient luminescent 10 screen for an electron tube and to provide a screen structure which will not burn or become discolored duringuse.

A further object of this invention is to provide a luminescent screen within an electron l6 discharge tube, which screen will be high insecondary electron emission when subject to the impact of an electron beam.

A further object of the invention is to provide a luminescent screen whose fluorescent efliciency 20 will increase with increases in primary electron velocities up to velocities of the order of 10,000 volts and higher.

My invention has as a further object that of providing screen material in which it is possible 25 to increase to a substantial extent the secondary electronic emission from the screen and, therefore, the candle power of the resulting luminescent spot measured with respect to the power input in the electron tube wherein the screen material is used.

Other. and further objects of the invention will become apparent and at once suggest themselves to those skilled in-the art to which the invention is directed by reading the following specification 35 and reierring to the single figure of the accompanying drawing which illustrates conventionally one form of the electron tube to which this invention may be applied.

By the following specification, it will be obvious that the screen material herein to be described may be applied to any type of electron tube wherein fluorescent screen structure is provided for causing light to be emitted upon a beam of electrons of high velocity bombarding or striking the same, the lightbeing initiated upon impact of the electron beam. However, for convenience of description, an electron tube of the cathode ray type now used to a considerable extent in connection with oscillograph ap- 50 paratus and cathode ray television systems will be referred to as a specific application of this invention.

Previous to this invention, many attempts have been made to increase secondary emission from I the screen structure when an electron beam im- -the tube.

pinges thereon, but these attempts have been unsatisfactory, and the screen has frequently become burned during the normal operation of The general effect of burning is thought to be due to a decomposition of the 5 fluorescent or luminescent material coating the inner tube wall upon which the electron beam impinges. Such burning manifests itself by a darkening of the screen structure and/or a decrease in its fluorescentproperties. According- 10 1y, by the present invention there has been described several methods of producing coating materials for electron tubes which have the desired property of increased secondary electron emission without any decided disadvantage of burnl6 ing. Briefly, such increase in secondary electron emission is believed to be due to the mixing of certain alkaline, or alkaline earth salts or compounds of metals of low work function, with the composition of materials from which the screen coating is normally developed. With such mixtures, a screen coating which has substantiallyidentical fluorescent properties, that is, the fluorescent efiects may be made visible upon impact of an electron beam within substantially the same portion of the visible spectrum, but the secondary emission characteristics of the material are accordingly increased. Such increase is believed to be obtained and is attributed to an intimate mixture of the added materials with the normally fluorescent crystals usually present in screen materials, for example, with the crystals of zinc silicate of the type known as "willemite Such intimate mixture may be either between the crystal boundaries or in the fluorescent crystals as a mixed crystal.

Referring now to the drawing accompanying this disclosure, cathode ray tubes of the general type used for oscilloscope and television purposes are generally of the form shown by the drawing. Such a tube is characterized by a long tubular neck I, one end of which carries the base 2, and the other portion of the tube is a conical portion 3 terminating in a section 4 whose normal projection is perpendicular to the axis of '45 the tube. The section 4 is concave on the inside and serves as a screen for the impinging pencil or beam of electrons developed within the tube. The beam or pencil of rapidly moving electrons has its origin in the heated cathode l4 located at the base end of the tube. As the electrons leave the cathode and enter the portion of the control grid, they are partially focussed into a beam by the control grid, and this control grid may also serve to modulate the beam in accordance with '05 limited to a small cross-section by a screen grid II and the cylindrical first anode i2, both of which may beheld at any suitable positive potential relative to the cathode by means of an external source of potential not shown. Electrons passing through the first anode I2 will be focussed into the form of a narrow beam and cause a small spot of light on the fluorescent screen 4 when such electron beam impinges upon the screen. Electrostatic deflection plates 6 and I in combination with electromagnetic coils, not shown, mounted aboutthe constricted section I3 may be employed to move the cathode ray in any desired manner, whereupon the cathode ray pencil traces a fluorescent pattern on the screen 5, which is in accordance wth the energized voltages or wave formations supplied to the beam defleeting plates and coils. It will be understood that the coils and plates deflect the developed electron beam in mutually perpendicular directions and that, in lieu of the combination of electrostatic and electromagnetic deflection for the beam, an all electrostatic system or an all electromagnetic system of deflection may be substituted. These types of deflecting systems are well known in the art and are not illustrated herein since the invention is not directlly con-- cerned with the manner in which the electron beam is deflected.

The second anode used in the tube consists of a conducting coating 8 upon the inner wall of the neck and conical section of the tube. A coating suitable for the inner wall of the tube to form a secondanode may be a conducting silver film or it may be a colloidal graphite film of the general type which is well known in the art or the coating may be in accordance with the disclosure in United States Patent 1,988,469. This internal conducting layer may conveniently be connected to an external source of positive voltage by means of a lead wire, not shown, con nected to a contact clip I 0. It is the purpose of this second .anode 8 to accelerate and together with the first anode ii to focus the developed stream or beam of electrons so that the developed stream or beam of electrons is focussed to a sharply defined spot and impinges at the desired velocity required to produce luminescence upon the specially treated screen 4. It has become customary in the art at present, and is highly desirable also, to deposit the fluorescentscreen material 4 directly upon the inner end wall of the bulb without providing electrical connections to the screen or without any conducting medium first being placed upon the tube wall. For this reason, secondary electron emission from the screen must be depended upon to discharge the screen with the impinging electrons. As was previously stated, the second anode located as described serves primarily as a means to accelerate and focus the-beam of electrons upon the screen, but this anode may also serve as a very convenient means for collecting the secondary electrons given ofl at the screen by the high velocity primary electrons of which the beam is composed. Thus any negative space charge which may develop around the luminous screen is effectively limited by the presence of the conducting coating of the second anode upon the inner conical wall of the bulb. A screen material of a nature suitable for the release of a substantial number of secondary electrons is as above stated one in which small percentages of certain normal alkaline or alkaline earth salts or compounds of metals of low work function are mixed with the fluorescent material used to coat the tube wall. The invention is applicable to screen material such as zinc orthosilicate (2ZnO,SiO2) which is often spoken of as "willernite. The preparation used to screen cathode ray tubes is in general not chemically pure willemite but contains mixed therewith a small percentage of magnesium orthosilicate (2MgO,SiOz) and a small amount of manganese oxide or silicate in solid solution. In general, chemically pure willemite will not fiuoresce satisfactory, but when traces of certain impurities such as manganese compounds are dissolved in it, it glows under electron bombardment with the result that a yellowish-green light which ceases almost instantly with cessation of the electron bombardment is developed.

While a screen of this type will have a short period of phosphorescence following electron bombardment, this factor, however, is satisfactory for television work because of the high frequency scanning used, and while in oscilloscope work it is frequently desirable to have a greater period of phosphorescence, this increased period of phosphorescence is easily obtainable by the addition of magnesium orthosilicate containing dissolved manganese. The magnesium orthosilicate is somewhat similar to the zinc orthosilicate characteristics except that the resultant glow or luminous effect will be closer to the red section of the spectrum and remain visible for a longer period of time after cessation of bombardment.

From the above discussion, it will be apparent that only extremely pure substances can be used in the preparation in order that control of the physical characteristics of the finished fluores-. cent material may be complete. Any spurious impurity may alter the characteristics sufliciently by destroying the efiects of those impurities deliberately introduced, as for example the introduction of manganese oxide into pure zinc orthosilicate. Consequently, in the following descriptions of preparations, it will be understood that the substances used in the preparations are to be as chemically pure as possible to give the most complete advantages pointed out above.

In accordance with the method to be herein described, one of' the preparations from which screen coating for electron tubes may develop may consist of the following elements arranged in accordance with the following table:

Material Weight Grams Zinc oxide 35 Silicon dioxide 19 Magnesium oxide 1 Boric acid l Barium carbonat 2 Manganese chloride 0.5 in 100 cc. H1O

the order of one hour, after which the sintered mass is removed from the furnace, cooled in air, and the resultant crystals are placed in a ball mill for grinding, so as to form a powdery mass. The above material may be referred to as barium willemite.

A screen material having generally similar properties to the above suggested mixture for forming a screen coating is one in which the process of manufacture is identical with: that material suggested above, but the barium carbonate (BaCOa) is'replaced by 5 grams of sodium carbonate (NazCOa). This material is prepared in the same manner in so far as the. drying, evaporating, heating, cooling and grinding is concerned. An additional type of screen material having general characteristics similar to that above suggested, which may be known as barium willemite is a material which is' herein identified as lithium willemite but wherein the barium carbonate (BaCOz) is replaced by 5 grams of lithium carbonate (LirCOs). A mixture of a nature quite similar to the above but which may be identified as thorium willemite is a mixture prepared from zinc oxide (Zn0)--70 grams, silicon dioxide (5109-38 grams, boric acid (H:BOs)-2 grams, magnesium oxide (MgO)-2 grams, manganese chloride 1 (MnCh.4H2O) 1 gram, and from the above mixture, grams is weighed out and mixed with thorium oxide (Th02)5 grams, and this material is wet mixed, and there is then added hydrofluoric acid (HF 48%) 20 cc. This mixtureis then dried as above suggested for the material which has been chosen to identify as barium .willemite, and is then fired at the temperature of 950 C. for a time period of the order of 45 minutes, after which the sintered mass is cooled and ground in a ball mill, as above described. Still another type of material is prepared in the same manner as above suggested in line with the material identified as lithium willemite except that the thorium oxide is replaced bycerium nitrate (Ce(Noa)a)5 grams. The mixture is then dried as above suggested for the material which it has been chosen to identify as barium willemite, and is then fired at the temperature of 950 C. for a time period of the order of 45 minutes after which the sintered mass is cooled and ground in a ball mill as above described.

Other mixtures which may be used, may be in accordance with the following formula: A mixture of zinc oxide (ZnO)-- grams; silicon dioxide38 grams, (S102); magnesium oxide iMgO)-2 grams; boric acid (H3BOs)-2 grams, of which 54 grams is used in the mixture, and a mixture of barium carbonate (BaCO3)1'l5 grams; silicon dioxide (SiOz)--38 grams, of which 5 grams is mixed with the 54 grams of the first mixture, and with this resultant mixture there is mixed manganese chloride (MnClz.4HzO)0.5 gram and cc. water (H20) and 20 cc. hydrofluoric acid (HF). All the foregoing materials are thoroughly mixed, dried, and then fired fora time period of the order of one hour at a temperature of the order of 1000 C.

A still further mixture which is satisfactory for the production of screen materials having high secondary emission properties may be formed according to the following formula: Zinc oxide (ZnO)-73.25 grams; silicon dioxide (SiO2) 35.14 grams; barium hydroxide (Ba(OH) 2.8H2O) 1.42 manganese chloride (mach 1.4mm- 1.04 grams, which materials are all mixed wet,

and to the mixture is added hydrofluoric acid (HF(46%)-18 ml. and the resultant material is dried, and then fired at a temperature of the order of 1000 C. for a time period of'the order of one hour.

From the foregoing, compounds may be used in the manufacturing process for screen materials having the characteristics above set forth. The firing temperatures for these mixtures are all of the order of 10Q0 C., and the time period of firing is of the order of one hour although in some instances the firing temperature may, be reduced slightly as is true particularly in the case of the mixtures identified herein as thorium and cerium willemite where the firing can be carried forward to the order of 950 C., but, in any event, the temperature of firing and the time limit for firing is not critical but is illustrated solely for the purpose of suggesting the time of firing and the temperature of firing in order to produce a material having satisfactory properties. It is, however, important that the addition of the alkaline or alkaline earth salts or compounds of metals of low work function as indicated in this specification be made prior to the firing operation because the addition of the materials to the mixtures prior to the firing is considered desirable in order to produce intimacy of contact of these salts with the zinc salts and, as previously stated, it is thought at this time that this is the condition which favors the increased secondary electron emission.

While the foregoing disclosure has suggested several different compositions of material used in the production of fluorescent screen materials having high secondary electron emitting properties under electron bombardment, it is, of course, readily apparent to those skilled in the art to which this invention is directed that a wide choice of materials may be made without departing from the spirit and scope of the disclosure and, therefore, it is believed that the claims herein after appended should be viewed in their broadest construction as embodying all materials selected from the general groupings hereinabove identified.

i Having now described the invention, what is claimed and desired to secure by Letters Patent is the following:

1. A fluorescent screen structure comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising zinc oxide, a manganese salt and a salt of an alkaline earth metal to produce a complex crystalline structure having high secondary electron emission characteristics.

2. A fluorescent screen structure comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising zinc oxide, a manganese salt and a salt of an alkaline metal to produce a complex crystalline structure having high secondary electron emission characteristics.

3. In an electron discharge device a fluorescent screen comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising a salt of barium and an alkaline earth metal to produce a' complex crystalline structure having high secondary electron emission characteristics.

4. A fluorescent screen structure comprising thereaction product resulting from the chemical and thermal treatment of a mixture comprising zinc 16 it will be seen that various.

oxide, a manganese oxide and a barium salt to produce a complex crystalline structure having high secondary electron emission characteristics.

5. In an electron discharge tube, an internal luminescent coating comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising a zinc oxide and a barium salt to produce a complex crystalline structure having high secondary electron emission characteristics.

6. A screen structure adapted to luminesce under electronic bombardment comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising a zinc oxide and a sodium salt to produce a complex crystalline structure having high secondary electron emission characteristics.

7, A screen structure adapted to luminesce under electronic bombardment comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising a zinc oxide and a lithium salt to produce a complex crystalline structure having high secondary electron emission characteristics.

8. A screen structure adapted toluminesce under electronic bombardment comprising the reaction product resulting from the chemical and thermal treatment of a mixture comprising a zinc oxide, a lithium salt, and a magnesium salt to produce a complex crystalline structure having high secondary electron emission characteristics.

LEWIS BARNARD HEADRICK. 

